The following technology has been known in respect of a conventional magnetic recording medium and a method of manufacturing thereof.
FIG. 10 is an outline views for explaining a hard disk as an example of a magnetic recording medium. In FIG. 10, FIG. 10(a) is a perspective view showing a total of a magnetic recording medium and FIG. 10(b) is a sectional view taken from a line A--A' of FIG. 10(a).
A substrate body 1 where a nonmagnetic (Ni(nickel)-P(phosphor)) layer 3 is provided on the surface of an Al(aluminum) substrate 2 is used. Further, on top of the substrate 1, a Cr (chromium) base layer 4, a ferromagnetic metal layer 5 and a protective layer 6 are laminated in this order.
The substrate body 1 is constructed in which the nonmagnetic (Ni--P) layer 3 is formed on the surface of the Al substrate 2 in a disk shape having a diameter of 89 mm (3.5 inch) and a thickness of 1.27 mm (50 mil) by a plating process or a sputtering process. Further, inscriptions (hereinafter, referred to as texture) in concentric shapes are provided on the surface of the nonmagnetic (Ni--P) layer 3 by mechanical polishing. Generally, the surface roughness, that is, the mean center line roughness Ra which is measured in the radial direction is 5 nm through 15 nm. Further, the Cr base layer 4 and the ferromagnetic metal layer 5 (generally, a magnetic film of Co (cobalt) alloy group), are formed on the surface of the substrate body 1 by a sputtering process and finally, the protective layer 6 comprising carbon or the like is provided by a sputtering process to protect the surface of the ferromagnetic metal layer 5. Typical thicknesses of the respective layers are, 5 .mu.m through 15 .mu.m for the nonmagnetic (Ni--P) layer 3, 50 nm through 150 nm for the Cr base metal layer 4, 30 nm through 100 nm for the ferromagnetic metal layer 5 and 20 nm through 50 nm for the protective layer 6.
The conventional magnetic recording medium having the above-described layer structure is fabricated under conditions of a back pressure at the order of 10.sup.-7 Torr of a film forming chamber before film formation by sputtering and an impurity concentration of 1 ppm or more of Ar (argon) gas used in the film formation.
It has been reported by Nakai et al. that according to the magnetic recording medium provided by the above-described fabrication process, especially in the case of the ferromagnetic metal layer 5 including Ta (tantalum) element (for example, CoCrTa alloy magnetic film), grain boundaries constructed of an amorphous structure are present among crystal grains forming the ferromagnetic metal layer and the grain boundaries comprise a nonmagnetic alloy composition (J. Nakai, E. Kusumoto, M. Kuwabara, T. Miyamoto, M. R. Visokay, K. Yoshikawa and K. Itayama, "Relation Between Microstructure of Grain Boundary and the Intergranular Exchange in CoCrTa Thin Film for Longitudinal Recording Media", IEEE Trans. Magn., vol. 30, No. 6, pp. 3969, 1994).
However, in the case of the ferromagnetic metal layer that does not include Ta element (for example, CoNiCr (chromium) or CoCrPt (platinum) alloy magnetic film), the above-described grain boundaries have not been confirmed.
Further, it has been described that when the ferromagnetic metal layer includes Ta element, the normalized coercive force (designated as Hc/Hkgrain) is provided with a value as large as 0.3 or more whereas it has a value smaller than 0.3 when the ferromagnetic metal layer does not include Ta element.
Further, International Application Publication No. PCT/JP94/01184 discloses a technology where in magnetic recording medium utilizing magnetic inversion in which a ferromagnetic metal layer is formed on the surface of a substrate body via a metallic base layer as an inexpensive high density recording medium having an increased coercive force without using an expensive ferromagnetic metal layer and a method of manufacturing thereof, an oxygen concentration of the metallic base layer and/or the ferromagnetic metal layer is decreased to 100 wtppm or below by decreasing an impurity concentration of Ar gases in film formation down to 10 ppb or below. Further, it has been reported that the coercive force is further increased by removing the surface of the substrate body by 0.2 nm through 1 nm by carrying out a cleaning treatment on the surface of the substrate body through a high frequency sputtering process by using Ar gas having an impurity concentration of 10 ppb or below before forming the metallic base layer. According to the report, it has been described that there is a correlation between a normalized coercive force and medium noise in a magnetic recording medium and the normalized coercive force should be 0.3 or more and lower than 0.5 to provide a low noise medium.
The normalized coercive force (Hc/Hkgrain) of a ferromagnetic metal layer is a value of a coercive force Hc divided by an anisotropic magnetic field Hkgrain of crystal grains, which represents a degree of enhancing magnetic isolation of crystal grains. That is, a high normalized coercive force of a ferromagnetic metal layer signifies that magnetic interaction of individual crystal grains constituting the ferromagnetic metal layer is lowered and a high coercive force can be realized.
Also, it has been known that a transitional region of magnetic inversion constitutes a noise source in respect of recording signals when a further high frequency recording is carried out in order to achieve high recording density. That is to say, when disturbance of the transitional region is large or when disturbance is caused in a wide range, there is a strong tendency of increasing noise whereby a magnetic recording medium having poor recording and reproducing property is formed.
In current magnetic recording media, certainly, a low noise medium is apt to be provided when the ferromagnetic metal layer is constituted by a CoCrTa alloy magnetic film and noise is apt to be enhanced when it is constituted by a CoNiCr or CoCrPt alloy magnetic film.
Meanwhile, with respect to a magnetic recording medium having a ferromagnetic metal layer comprising a CoCrTa alloy magnetic film, it is difficult to fabricate stably a medium having a high coercive force in mass production since the film is liable to suffer influence of an atmosphere of film formation. By contrast, a CoNiCr or CoCrPt alloy magnetic film has an advantage that the coercive force in mass production is obtained comparatively stably.
Accordingly, in respect of the magnetic recording medium having the ferromagnetic metal layer comprising a CoNiCr or CoCrPt alloy magnetic film whereby the coercive force in mass production can be obtained comparatively stably, a magnetic recording medium having a property of a high S/N ratio (recording signal S, medium noise N) in the electromagnetic conversion property and a method of fabricating thereof have been desired to realize.
It is a first object of the present invention to provide a magnetic recording medium in which in respect of a magnetic recording medium having a ferromagnetic metal layer comprising a CoNiCr or CoCrPt alloy magnetic film, the S/N ratio (recording signal S, medium noise N) of the electromagnetic conversion property is high and the coercive force in mass production can stably be obtained.
Further, it is a second object of the present invention to provide a method of manufacturing a magnetic recording medium capable of easily forming a medium having a coercive force with low surface temperature of a substrate body during a film formation, and also with no electric bias application with respect to a substrate body.